Tuesday, May 15, 2012

The word “prokaryote” in microbiology: a convenient classification term covering up knowledge gaps

The word prokaryote, sometimes spelled procaryote, is composed of the Greek roots pro and karyon for “before” and for “nut,” “seed” or “grain,” respectively. Within cytology contexts,  “grain” refers to the nucleus of a cell. Prokaryote literally means “before a nucleus,” describing a cell with no nucleus [1]. In contrast, an organisms consisting of cells with a nucleus is called eukaryote. The Greek prefix eu refers to a normal or well-composed condition.

The distinction between eukaryotic and prokaryotic cellular systems was first made in 1937 by the French biologist Edouard Chatton (1883-1947), who significantly contributed to our knowledge of single-celled protoctists such as ciliates and dinoflagellates and of mitotic cytology [2,3].

The paradigm of a simple prokaryote-eukaryote division has now been broken. In his book The Third Domain Tim Friend writes [3]: “Prokaryotes are a fabrication. They do not exist.” Friend reports that microbiologists Carl Woese and Norman Pace, known for their work on microorganism classification based on microbial RNA studies, “wish to rid microbiology entirely of the term prokaryote." This term was created as a matter of convenience and does not reflect how most current biologists depict the tree of life with its three main branches (domains) archaea, bacteria and eukarya (eucarya). Everything “non-eukarya” should not carelessly dumped into the “prokaryote basket”—and it doesn't have to with ever more sophisticated tools for molecular taxonomy becoming available.

Established terms rarely disappear completely.  The Principles of Modern Microbiology by Mark Wheelis [4], for example, surveys “procaryotic microbes” and focuses on the diversity of bacteria and archaea, introducing groups and lineages such as green-sulfur, green-nonsulfur, purple-nonsulfur, aerobic-sulfur and sulfate-reducing bacteria, deinococci, proteobacteria, gram-positive bacteria, cyanobacteria, spirochetes, chlamydia, euryarchaeotes, crenarchaeotes and nanoarchaeotes, just to name a few. Obviously, biodiversity is not a matter of having or not having a cell nucleus.  

Keywords: microbiology, prokaryote-eukaryote dichotomy, cell biology concepts, super-kingdoms, taxonomy, nomenclature.

References and more to explore
[1] wiseGEEK: What Are Prokaryotic Cells? [www.wisegeek.com/what-are-prokaryotic-cells.htm].
[2] Marie-Odile Soyer-Gobillard: Edouard Chatton (1883-1947) and the dinoflagellate protists: concepts and models. International Microbiology 2006, 9, pp. 173-177 [www.im.microbios.org/0903/0903173.pdf].
[3] Tim Friend: The Third Domain. The Untold Story of Archaea and the Future of Biotechnology. Joseph Henry Press, Washington, D.C., 2007; pp. 60-61 and 75.
[4] Mark Wheelis:  Principles of Modern Microbiology. Jones and Bartlett Publishers, Sudbury, Massachusetts, 2008; pp.305-321.

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